1. Field of the Invention
The present invention relates to antennas and more particularly to meander line antennas.
2. Brief Description of Prior Developments
In the past, efficient antennas have typically required structures with minimum dimensions on the order of a quarter wavelength of the radiating frequency. These dimensions allowed the antenna to be excited easily and to be operated at or near a resonance, limiting the energy dissipated in resistive losses and maximizing the transmitted energy. These antennas tended to be large in size at the resonant wavelength.
Further, as frequency decreased, the antenna dimensions increased in proportion. In order to address the shortcomings of traditional antenna design and functionality, researchers developed the meander line loaded antenna. One such is disclosed in the U.S. Pat. No. 5,790,080 for Meander Line Loaded Antenna, which is hereby incorporated herein by reference. This patent describes an antenna that includes one or more conductive elements for acting as radiating antenna elements, and a slow wave meander line adapted to couple electrical signals between the conductive elements. The meander line has an effective electrical length that affects the electrical length and operating characteristics of the antenna. The electrical length and operating mode of the antenna is readily controlled.
Meander lines are connected between the vertical and horizontal conductors at the gaps. The meander lines are designed to adjust the electrical length of the antenna. In addition, the design of the meander slow wave structure permits lengths of the meander line to be switched in or out of the circuit quickly and with negligible loss, in order to change the effective electrical length of the antenna. This switching is possible because the active switching devices are always located in the high impedance sections of the meander line. This feature keeps the current through the switching devices low and results in very low dissipation losses in the switch, thereby maintaining high antenna efficiency.
The basic antenna can be operated in a loop mode that provides a xe2x80x9cfigure eightxe2x80x9d coverage pattern. Horizontal polarization, loop mode, is obtained when the antenna is operated at a frequency such that the electrical length of the entire line, including the meander lines, is a multiple of full wavelength. The antenna can also be operated in a vertically polarized, monopole mode, by adjusting the electrical length to an odd multiple of a half wavelength at the operating frequency. The meander lines can be tuned using electrical or mechanical switches to change the mode of operation at a given frequency or to switch frequency using a given mode.
The meander line loaded antenna allows the physical antenna dimensions to be reduced significantly while maintaining an electrical length that is still a multiple of a quarter wavelength of the operating frequency. Antennas and radiating structures built using this design in the region where the limitation on their fundamental performance is governed by the Chu-Harrington relation:
xe2x80x83Efficiency=FV2Q
where:
Q=Quality Factor
V2=Volume of the structure in cubic wavelengths
F=Geometric Form Factor (F=64 for a cube or a sphere)
Meander line loaded antennas achieve the efficiency limit of the Chu-Harrington relation while allowing the antenna size to be much less than a wavelength at the frequency of operation. Height reductions of 10 to 1 can be achieved over quarter wave monopole antennas, while achieving comparable gain.
U.S. Pat. No. 6,323,814 for Wide Band Meander Line Loaded Antenna is also incorporated herein by reference. This reference discloses a meander line loaded antenna which provides a wide instantaneous bandwidth. A first planar conductor is substantially parallel to the ground plane and is separated from the first planar conductor by a gap. A meander line interconnects the first and second planar conductors. The antenna may be arranged in opposed pairs, and also as two orthogonally opposed pairs for enabling circular polarization.
The present invention is a dual band/dual mode meander line antenna which includes a ground plane and a pair of substantially vertical radiating surface elements disposed substantially parallel to one another and perpendicular to the ground plane. A generally horizontal top plate element is in substantially parallel relation to the ground plane. A wide band meander line element is capacitively connected to the horizontal top radiating element. At least one narrow band meander line element connected to the top horizontal radiating element.